The invention relates to an ICR cell operating with a duplexer comprising one or more semiconductor components for use in a device for Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry comprising a preferably superconducting magnet for generating a magnetic field in the direction of a z axis, wherein the duplexer is an integral part of a transmission and receiving device of an FT-ICR mass spectrometry device, which, on the one hand transmits the voltage of the transmitter during an ion excitation phase over the transmitter path of the duplexer to at least one electrode of the ICR cell and protects a preamplifier from overvoltage by antiparallel diodes and a serial impedance for current limiting and, on the other hand, transmits an ion received signal, namely the voltage of the same electrode following from the influenced charge, via a receive path to the preamplifier during an ion detection phase.
Such an arrangement is known from Chen, T.; Kaiser, N. K.; Beu, S. C.; Hendrickson, C. L. and Marshall, A. G., Excitation and Detection with the Same Electrodes for Improved FT-ICR MS Performance, Proc. 60th ASMS Conf. on Mass Spectrometry & Allied Topics, Vancouver, Canada, May 20-24, 2012 (=reference [2])
or from
Chen, T.; Kaiser, N. K.; Beu, S. C, Blakney G. T., Quinn J. P., McIntosh, D. G., Hendrickson, C. L. and Marshall, A. G., Improving Radial and Axial Uniformity of the Excitation Electric Field in a Closed Dynamically Harmonized FT-ICR Cell, 61st Amer. Soc. Mass Spectrometry Conf., Minneapolis, Minn., Jun. 9-13, 2013 (=reference [2]).
Fourier transform ion cyclotron resonance (FT-ICR) is a technical method for high resolution mass spectrometry.
Customary cells used for FT-ICR mass spectrometry are divided into cubic and cylindrical geometries: one pair of opposing electrodes for ion excitation, and another pair, offset by 90 degrees, for detection, as shown by way of example in FIG. 2 (or FIG. 3a). A refinement attempts to improve this existing arrangement by using all electrodes for ion detection, more particularly by using the electrode pair previously used only for excitation also for detection.
By adding the signals of all four electrodes having a respective alternating phase (0 degrees, 180 degrees), a higher frequency resolution is achieved (actually, a higher frequency is achieved; in FT-ICR mass spectrometry, this corresponds to a higher mass resolution). This detection type is known by the term harmonic detection method (FIG. 3b) (see reference [9]).
However, such an arrangement can also be used to achieve greater sensitivity (higher signal-to-noise ratio) by way of in-phase addition of the signals since an ion received signal is detectable during the entire orbit (cyclotron). The respective signals of two adjoining electrodes are added, the signals of the two other electrodes are subtracted (FIG. 3c) (see reference [8]).
A basic diagram of this known arrangement of the electrode pairs is shown in FIG. 4a. A spatially opposing electrode pair (20 and 21) of an ICR cell (01), together with the associated preamplifiers (04b and 04d), is used only for detection, while the second electrode pair (40 and 41) is connected either to the preamplifiers (04a and 04c) or the transmitters (03a and 03b, shown as two individual transmitters here; however, in practice, often a single transmitter comprising a 0/180 degree splitter is used) via the duplexers (08a and 08b) for the ion excitation. This arrangement results in four freely combinable receive paths and two transmission paths for various applications.
A single path, comprising a shared electrode (11) for excitation and detection, is shown in FIGS. 4b and 4c for the excitation and detection case. A single duplexer from FIG. 4a (08a or 08b) is substantially composed of two circuit paths S1 and S2 (FIGS. 4b and 4c, 42 and 43). S1 (42) is closed, respectively in a conducting state, and S2 (43) is opened, respectively in a non-conducting state, during the ion excitation phase, and the states are reversed during the ion detection phase.
In the closed state, S1 transmits the ion excitation voltage to the shared electrode, and in the non-conducting state it ensures that the detected ion received signal is not attenuated. In the non-conducting state, S2 protects the downstream preamplifier from the high ion excitation voltage, and in the conducting state it transmits the ion received signal.
The objective of such an arrangement is to achieve a signal-to-noise ratio as high as possible, and/or a frequency resolution as high as possible, without impairing or limiting any other system properties to the extent possible. The most important aspects that must be met by the application are listed below:                1. So as to achieve a higher frequency resolution (harmonic detection method, FIG. 3b), at least one electrode pair must be designed for transmitting and receiving, and the ion received signals must be appropriately combined.        2. So as to maximize the signal-to-noise ratio during the ion detection phase, the conducting behavior of S2 (43, preamplifier protection during the ion excitation phase, FIGS. 4b and 4c) must be as ideal as possible.                    In addition, a potentially present capacitance from the receive path (12) to circuit ground (13) must be minimized, and a potentially present parallel resistance to circuit ground must be maximized.                        3. So as to ensure protection of the preamplifier during the ion excitation phase, S2 must have a sufficiently high isolation and input/output isolating voltage.        4. So as to maximize the signal-to-noise ratio during the ion detection phase, the isolation of S1 (42, transmission of the ion excitation voltage to the shared electrode (11), FIGS. 4b and 4c) must be as ideal as possible.        5. In the conducting state, the resistance of S1 (FIGS. 4b and 4c), together with the ICR cell capacitance (FIG. 5, detail 51), forms a low-pass filter and accordingly must be low resistive so as not to influence the frequency response of the ion excitation voltage.        6. The duplexer, together with the circuit paths S1 and S2 thereof, must be able to change sufficiently quickly between the two basic states so that the functionality of a changeover switch between excitation and detection is ensured.        
The most important aspects that must be met for a specific implementation are listed below:                1. The main problem of the implementation lies in the highly resistive source impedance of the ICR cell, which necessitates a preamplifier having a minimal equivalent noise current source. The duplexer must not burden this highly resistive system in an interfering manner (FIG. 5).        2. If the preamplifier protection is implemented by way of a switched path S2 (FIGS. 4b and 4c), the actuation of the switch must be ensured under all circumstances so as to protect the preamplifier from the ion excitation voltage.        3. So as to be able to utilize the improved properties of an ICR cell comprising a shared electrode pair for ion excitation and detection, it is advantageous for the behavior of the downstream preamplifier to be as low-noise as possible and matched to the source impedance of the cell. The term “noise matching” is often used in the literature for this behavior.        
The electronic circuit published in the reference [1] describes in great detail the current state of preamplifier technology for FT-ICR mass spectrometry as it is often used today, however without a duplexer. This paper clearly reveals which parameters are essential for a preamplifier design. It is derived in detail that the total input capacitance (51), composed of the electrode capacitance, the feed capacitance to the preamplifier, the input capacitance of the preamplifier, and further parasitic capacitances, must be minimized to achieve a maximal signal-to-noise ratio, while the total parallel resistance (52), which in turn is composed of the input resistance of the preamplifier, the resistance to ground for electrode DC potential (10) and further parallel losses, must be maximized.
The best signal-to-noise ratio possible using current technologies (apart from a conceivable cryogenic preamplifier, which could be used to reduce the noise even further) can undoubtedly be achieved from a single electrode pair by way of such an arrangement. However, this system can only be used for ion detection since the other electrode pair is needed for ion excitation, which accordingly precludes certain applications, such as the harmonic detection method and/or further increases in sensitivity by way of in-phase combination of the received signals (see reference [8]).
FIG. 2 shows this existing prior art according to reference [4]. This general composition of a conventional ICR cell, as it is used in the majority of commercially available FT-ICR mass spectrometry devices, includes two electrodes (22 and 23) for ion excitation and two electrodes (20 and 21) for ion detection. The ion excitation voltage is provided by two transmitters (03a and 03b, which are shown as two individual transmitters here; however, in practice often a single transmitter comprising a 0/180 degree splitter is used), and the detected ion received signal is typically amplified by two preamplifiers (04a and 04b, shown as two preamplifiers here, but usually implemented as a single preamplifier having a differential input) in a manner that is as low-noise as possible.
In an ICR cell comprising a shared electrode pair for ion excitation and detection, the preamplifier protection is added to the minimization of the total input capacitance and the maximization of the total parallel resistance. Few articles have been published that address this topic. Hereafter, the features of the circuit published in references [2] and [3] (FIG. 6) are described. A distinction is made between the implementation for circuit paths S1 and S2 (FIGS. 4b and 4c, 42 and 43).                a) S1: All known implementations of the described principles in FIGS. 4b and 4c have in common that an anti-parallel diode pair (FIG. 6, detail 05) is used for S1 (42).                    The ion excitation voltage is several times greater than the diode forward voltage, and any half wave is able to pass the diode almost loss-free. In contrast, the detected ion received signal is several times smaller than the diode forward voltage, and the diodes act as a blocking switch for the signal.                        b) S2: So as to protect the preamplifier from the ion excitation voltage, a voltage divider is used, composed of a reactance connected in series with the preamplifier input (this is a series capacitor in the published variant, see FIG. 6, detail 60) and multiple anti-parallel diode pairs (FIG. 6, detail 06 from reference [2]) in parallel with the amplifier input. The diode pairs limit the maximum alternating voltage present at the preamplifier input during the phase of ion excitation. The current in the arrangement is determined by the dimensioning of the series capacitor (numerical example based on the following assumptions: 200 m/z mass-to-charge ratio, 21 Tesla magnet, frequency of the ion excitation voltage approximately 1.6 MHz having a peak voltage of 200 V. At a series capacitance of 1 nF, a peak current of almost 2 A flows in the series capacitor, or approximately 1 A per diode). Limiting the current by way of a capacitor has the advantage that the reactance of a capacitor does not have noise, in comparison with an equally large real resistor. Depending on the selection of this capacitor, this arrangement has the following properties:                    a. The maximum achievable signal-to-noise ratio during the ion detection phase is heavily influenced by a further voltage divider, composed of the series capacitance (60), the parasitic capacitances of the diode pairs (numerical example: 4× CD@0V of approximately 1.5 pF results in 6 pF) and the parasitic input capacitance (numerical example: Ci approximately 10 pF) of the preamplifier (combined as Cp in 61).                            A small value of the series capacitance means a high reactance and thus reduces the necessary ampacity of the diodes in parallel with the amplifier input (ion excitation phase), but at times divides the detected ion signal down drastically, thereby worsening the signal-to-noise ratio achievable by the arrangement (ion detection phase).                                    b. At a high value of the series capacitance (60), the resulting voltage divider practically has no influence on the maximum achievable signal-to-noise ratio. In return, a much higher current flows through the diode pairs (06) during the ion excitation phase. To ensure a reliable operation, diodes must be selected which are designed for higher current, or the higher current must be divided to even more diode pairs. Diodes having a higher ampacity have a larger chip surface, and hence a larger parasitic capacitance (low-frequency diodes small-signal model in FIG. 7, detail 73). At the same time, the parasitic diode parallel resistance (70) also decreases. Both result in the maximum achievable signal-to-noise ratio being reduced.                            A distribution of the higher current to a larger number of diode pairs (see reference [2]) has the same effect since the entire chip surface of all diodes increases.                                                
A further feature of the circuit published in references [2] and [3] is the resistance to ground for electrode DC potential (FIG. 6, detail 10) of the electrode (11), shared for excitation and detection. The resistance to ground discharges potential electrical charges from the electrode and generates the DC reference potential for the ICR cell and advantageously is selected as highly resistive as possible for the signal-to-noise ratio.
It is the object of the present invention to provide a duplexer for an ICR cell of an FT-ICR mass spectrometry device in which at least one electrode can be used for both ion excitation and then for ion detection, wherein the duplexer used for this purpose ensures the protection of the preamplifier from the excitation voltage and does not significantly impair the signal-to-noise ratio.